1. Field of the Invention
The present invention relates to a high-frequency switch, particularly, it relates to a high-frequency switch and more for switching a signal path in a high-frequency circuit of a digital portable telephone and the like.
2. Description of the Prior Art
As shown in FIG. 13, a high-frequency switch is used for switching the connection between a transmitting circuit TX and an antenna ANT, and the connection between a receiving circuit RX and the antenna ANT in a digital portable telephone and the like. As shown in FIG. 14, the high-frequency switch 1 is connected to the antenna ANT, transmitting circuit TX and receiving circuit RX. An anode of a first diode 3 is connected to the transmitting circuit TX via a capacitor 2a. The anode of the first diode 3 is grounded via a series circuit of a first strip line 4 and a capacitor 2b. A control terminal 6 is connected to an intermediate point between the first strip line 4 and the capacitor 2b via-a resistor 5. A control circuit for switching the high-frequency switch 1 is connected to the control terminal 6. A cathode of the first diode 3 is connected to the antenna ANT via a capacitor 2c.
The receiving circuit RX is connected to the capacitor 2c connected to the antenna ANT via a series circuit of a second strip line 7 and a capacitor 2d. An anode of a second diode 8 is connected to an intermediate point between the second strip line 7 and the capacitor 2d. A cathode of the second diode 8 is grounded.
In the case of transmission using the high-frequency switch 1, a positive voltage is applied to the control terminal 6. The first diode 3 and the second diode 8 are turned ON by this voltage. It is so arranged that, a DC current is blocked by the capacitors 2a, 2b, 2c and 2d, so that the voltage applied to the control terminal 6 is applied only to a circuit including the diodes 3 and 8. Since the diodes 3 and 8 have been turned ON, a signal from the transmitting circuit TX is sent to the antenna ANT and transmitted from the antenna ANT. Since the resonance is given at a frequency of the transmission signal of the transmitting circuit TX by grounding the second strip line 7 by the second diode 8 and its impedance becomes infinite, the signal is not transferred to the receiving circuit RX. At the time of reception, the first diode 3 and the second diode 8 are turned OFF by not applying the voltage to the control terminal 6. And hence, a receiving signal is transferred to the receiving circuit RX and is not transferred to the transmitting circuit TX. As such, by controlling the voltage applied to the control terminal 6, the transmission and reception can be switched.
However, in a diode used in the high-frequency switch, complete conduction can not be attained when the diode is ON, and inductance exists. Though the transmitting circuit and the antenna are in conduction when the diode is ON, due to the inductance of the second diode, an impedance from a contact point A of the antenna and the second strip line to the receiving circuit RX side is not infinite, and hence the signal from the transmitting circuit leaks to the receiving circuit side, and as shown in FIG. 15, an insertion loss between the transmitting circuit and the antenna becomes larger. Also, as shown in FIG. 16, isolation between the antenna and the receiving circuit can not be secured.
When the diode is OFF, capacitance exists in the diode and complete insulation can not be attained. When the diode is OFF, though the antenna and the receiving circuit are in conduction, the reception signal from the antenna leaks to the transmitting circuit due to the capacitance of the first diode, and as shown in FIG. 17, isolation between the transmitting circuit and the antenna can not be secured. As shown in FIG. 18, since the reception signal leaks to the transmitting circuit side, an insertion loss between the antenna and the receiving circuit becomes larger. As such, due to the inductance or capacitance existing in the diode, the performance of the high-frequency switch is deteriorated.